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The electrical engineering is that field of the technique that deals with the study and practical applications of
electricity. In particular, the specialized field in production, transportation, distribution and use of electric
energy. The Electricity is a secondary source of energy because is obtained from the conversion of other kind of
energy, called primary (chemical, mechanical). Electricity is produced in two forms: direct
current (DC), and alternating current (AC). The second was prefered, in
large scale use, for its ease of processing and transportation. Its characteristics limit losses due to the Joule
effect, so it's more efficient. The Joule effect is the phenomenon that procduce heating when in a conductors flow
electric current. That is due for the opposition of the flow by its electrical resistance (specific for each material
used). To clarify the concept it's necessary introduce one of the basic electrical engineering's laws,
the Ohm's law.
It tells us that the difference of potential measured at the ends of a conductor is the value obtain by the
product of conductor's resistance by the current intensity circulating in it (direct current).
V = R * I by unit misure
Volt = Ohm * Ampere by symbols
V = Ω * A
In few words, we know that Power dissipation is
(remember direct current) by the product of voltage by
the flowed current, briefly:
P = V * I by unit misure
Watt = Volt * Ampere by symbols
W = V * A
substituting V with R * I we also have :
P = R * I * I so P = R * I2
As you can see from the formula the power dissipated is proportional to the resistance,
but it is also proportional to the square of current flowing! Since the electrical resistance
below a certain value can not be limited by mechanical issues, it is far preferable to limit
the current intensity flowing. To do this, the transportation voltage is risen proportionally.
For istance:
P = 220 * 5 = 1100W - P = 1100 * 1 = 1100W
Now assuming we have a total conductors electrical resistance of 10Ω in the
first case we will have a heat dissipation of 250W, while in the latter that
corresponds to only 10W. Said in percentage terms, a loss of
22.73%, against 0,91%.
In order to raise or lower the voltage we use a machine called transformer,
which, however, works only with an alternating current. The electrical transformer is a static machine, means that
has no moving parts, one of the most efficient man-made. In fact, its performance, depending on construction
characteristics and materials used, can be up to 99%. The Joule effect or heat dissipation, exist also in
alternating currents, but to be able to calculate it we must be introduce the other variables, typical of alternating
current.
Said that, I hope I have cleared the reason for the choice of alternating current electricity usage, which we all have
available in our homes.
Finally, a good example of using direct current electricity is represented by the railway, which uses the nominal 3000V DC
to run the motors of electric locomotives.
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electricity